The invention relates to the manufacture of hot-rolled and cold-rolled sheet from iron-carbon-manganese austenitic steels having very high mechanical properties, and especially a high mechanical strength combined with excellent resistance to delayed cracking.
In view of fuel economy and safety in the case of collisions, high strength steels are more and more used in the automobile industry. This requires the use of structural materials that combine a high tensile strength with high ductility. To meet these requirements, patent FR 2 829 775 discloses for example austenitic alloys having as main elements iron, carbon (up to 2%) and manganese (between 10 and 40%) which can be hot-rolled or cold-rolled and have a strength that may exceed 1200 MPa. The mode of deformation of these steels depends on the stacking fault energy: for a sufficiently high stacking fault energy, the observed mode of mechanical deformation is twinning, which results in a high work hardenability. By acting as an obstacle to the propagation of dislocations, the twins increase the flow stress. However, when the stacking fault energy exceeds a certain limit, slip of perfect dislocations becomes the main deformation mechanism and the work hardening is reduced. The patent mentioned above discloses Fe—C—Mn steels whose stacking fault energy is such that a high work hardening is observed combined with a very high mechanical strength. Furthermore, it is known that the sensitivity to delayed cracking increases with the mechanical strength, in particular after certain cold-forming operations since high residual tensile stresses are liable to remain after deformation. In combination with atomic hydrogen possibly present in the metal, these stresses are liable to result in delayed cracking, that is to say cracking that occurs a certain time after the deformation itself. Hydrogen may progressively build up by diffusion to crystal lattice defects, such as matrix/inclusion interfaces, twin boundaries and grain boundaries. It is in the latter areas that hydrogen may become harmful when it reaches a critical concentration after a certain time. For a constant grain size, the time required to attain a critical level depends on the initial concentration of mobile hydrogen, the intensity of the residual stress concentration field and the kinetics of hydrogen diffusion.
In particular circumstances, small amounts of hydrogen may be introduced at some stages of steel fabrication such as chemical or electrochemical pickling, annealing under special atmospheres, electroplating or hot dip galvanizing and during Plasma Vacuum Deposition (PVD). Subsequent machining operations using lubricating oils and greases may be also a cause for hydrogen production after decomposition of these substances at high temperatures.
For example, delayed cracking may be encountered in the fabrication of bolts made out medium-carbon steels, which includes a cold forging step. U.S. Pat. No. 6,261,388 discloses cold forging steels for the fabrication of wires and bars for bolts, gears or shafts. The main elements of the composition are: C: 0.1-0.4%, Mn: 0.3-1%, Si<0.15%, Cr: 0.5-1.2%, B: 0.0003-0.005%, Ti: 0.020-0.1.00% and the matrix contains fine Ti or Nb carbonitrides for limiting grain coarsening. Good resistance to delayed cracking of steels with an ultimate tensile strength (UTS) of 1000-1400 MPa is obtained by forming a dense scale enriched in Cr, thereby increasing corrosion resistance and thus reducing the amount of hydrogen produced in the process of corrosion. Reduction of sulphur and phosphorus were also found as solutions to increase delayed cracking resistance. However, these solutions address quenched and tempered steels whose microstructure totally differs from the fully austenitic steels which will be considered here.
Furthermore, it is known that, according to the level of steel resistance, annealing treatments may be performed to reduce the sensitivity to delayed cracking: ISO standard 2081-1986 related to electrolytic deposits on iron and steel defines annealing treatments on high strength martensitic steels for bolts: annealing temperature θ and holding times t increase with steel resistance. For the most resistant steels, annealing treatments with θ=150-220° C., t=24 h, causing hydrogen diffusion, are recommended.
However the document indicates that these treatments are not applicable to coatings applied to sheets or strips in the unfabricated form. Moreover, these treatments address medium carbon martensitic steels with low ductility and not the austenitic Fe—C—Mn alloys mentioned above, whose compositions are totally different. It is also known that the hydrogen diffusion coefficient is very different in austenite when compared to martensite.